Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. In the field of semiconductor metrology, a metrology tool includes an illumination system which illuminates a target, a collection system which captures relevant information provided by the illumination system's interaction (or lack thereof) with a target, and a processing system which analyzes the information collected using one or more algorithms. Metrology tools can be used to measure structural and material characteristics (e.g., material composition, dimensional characteristics of structures and films such as film thickness and/or critical dimensions of structures, overlay, etc.) associated with various semiconductor fabrication processes. These measurements are used to facilitate process controls and/or yield efficiencies in the manufacture of semiconductor dies.
Traditionally, measurement targets are designed to fit within a scribe line between die on a semiconductor wafer. However, more recently, measurement targets are designed to fit within the die area. As measurement targets decrease in size, the size of the illumination spot projected onto the specimen should also decrease. The size of the illumination spot must be small enough to ensure that the detected light-sample interactions originate within the measurement target of interest and are not contaminated by light that interacts with structures outside the region of interest.
Traditionally, metrology systems are designed to optimize the illumination profile projected onto the specimen by appropriate optical design in both the illumination and collection paths (i.e., magnification, apodization, diffraction control, etc.). Unfortunately, these techniques generally reduce the amount of light projected onto the specimen. For example, magnification reduces the solid angle of collection from the source. Due to the conservation of etendue, the usable spectral power density from the source is also reduced. In another example, apodization tailors the source spatial intensity distribution projected onto the specimen to reduce contamination from outside the region of interest. In yet another example, field stops reduce the lateral extent of the source that is imaged onto the specimen. Each of these measures inherently reduces the brightness of the illumination system.
In some examples, longer integration times are used to accommodate the light lost due to traditional optical design features employed to reduce the illumination spot size (e.g., magnification, apodization, etc.). However, longer integration times increase the system sensitivity to lower frequency noise sources. In some examples, low frequency noise sources become dominant and further increases in integration time no longer improves the system signal to noise ratio. In addition, the increase in signal acquisition time is not desirable from a product perspective as it reduces system throughput and thus increases cost of ownership of the system.
In some examples, current metrology systems employ an electrode-based, relatively high intensity discharge arc lamp to generate illumination light. These arc lamps include an anode and cathode to excite a working gas (typically xenon or mercury gas) contained within a chamber of the lamp. An electrical discharge is generated between the anode and cathode to provide power to the excited (e.g., ionized) gas to sustain light emission from the ionized gas during operation of the light source. However, these light sources have a number of disadvantages. For example, electrode based, relatively high intensity discharge arc lamps have radiance limits and power limits due to electrostatic constraints on current density from the electrodes, the limited emissivity of gases as black body emitters, the relatively rapid erosion of electrodes made from refractory materials due to the presence of relatively large current densities at the cathodes, and the inability to control dopants (which can lower the operating temperature of the refractory cathodes) for relatively long periods of time at the required emission current. As a result, state of the art xenon-based arc lamps typically generate light with a color temperature that is limited to approximately 6,000 degrees Kelvin.
The use of electrodes limits the amount of energy that can be transferred to the plasma as there is a finite surface area over which energy is transferred from the electrode to the plasma. The proximity of the electrode to the plasma results in electrode surface bombardment by highly energetic particles. This causes sputtering of the electrode material that leaves deposits of the electrode material on the surface of optical components. This reduces transmission and the radiance of the illumination delivered to the metrology system. In addition, the proximity of the electrode to the plasma increases the electrode temperature. This may alter the material properties of the electrode, which, in turn, influences plasma properties.
In some other examples, laser sustained plasma based light sources have been developed. An exemplary laser sustained plasma system is described in U.S. Pat. No. 7,786,455 assigned to Energetiq Technology Inc. In one example, a xenon lamp is ignited with a high voltage pulse applied through electrodes. Once started, the plasma is continuously sustained with an energetic laser beam focused to a small volume inside the Xenon gas envelope. However, laser sustained plasma light sources also face significant limitations. The laser sustained plasma develops a temperature gradient across the plasma (i.e., hotter on the inside, cooler on the outside) that causes self-absorption; particularly absorption of short wavelength light. As a result, it is difficult to increase the plasma temperature beyond approximately 12,000-15,000 degrees Kelvin. Moreover, an increase in laser power generally results in a larger plasma size that has a diminishing impact on color temperature and radiance.
In some other examples, light sources employing laser produced plasma have been developed for lithographic applications. An exemplary extreme ultraviolet (EUV) light source is described in U.S. Pat. No. 7,368,741 assigned to Cymer, Inc. In one example, a working gas at low pressure is pre-ionized by a radiofrequency coil, ignited with a focused laser beam, and sustained by a combination of the focused laser beam and electrical discharge to generate a pinch plasma that emits EUV light. Generation of illumination suitable for EUV lithography requires emission along atomic lines, rather than broadband emission. As a result, EUV illumination sources generate plasma in a low pressure (i.e., vacuum) environment to minimize reabsorption of the EUV emission. The low pressure environment allows the plasma to spread over a large volume. If relatively high pressures were employed (e.g., 0.5 atmosphere, or greater), these sources would fail to generate any useable amount of EUV light. Large plasma volumes are acceptable for an EUV source that focuses on the generation of as much EUV emission along atomic lines as possible. However, in a metrology application, high radiance, broadband radiation is required. The large plasmas generated by systems designed for EUV emission suffer from low brightness and narrow band emission that does not fulfill the requirement for broadband, high brightness illumination in metrology applications.
Existing illumination sources are limited in radiance and color temperature for small spot size metrology applications. Thus, improved methods and systems for generating and extracting high radiance, broadband light at suitable flux levels are desired.